refactor: Replace 'meta.ci' -> 'meta.ci.skip'

We need 'meta.ci' to be an attribute set for new CI features.

Change-Id: I83d04e2d74e42e49fe739b049ee4ba799f6d5d86
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5007
Tested-by: BuildkiteCI
Reviewed-by: ezemtsov <eugene.zemtsov@gmail.com>
Reviewed-by: wpcarro <wpcarro@gmail.com>

third_party/overlays/tvl.nix

@@ -47,6 +47,6 @@ self: super: {
 
   # Avoid builds of mkShell derivations in CI.
   mkShell = super.lib.makeOverridable(args: (super.mkShell args) // {
-    meta.ci = false;
+    meta.ci.skip = true;
   });
 }